The activation of unmyelinated or myelinated afferent fibers by brief infrared laser pulses varies with skin type

Small nerve fiber selectivity of laser and intraepidermal electrical stimulation: A comparative study between glabrous and hairy skin

OBJECTIVES

In clinical neurophysiology practice, various methods of stimulation can be used to activate small-diameter nociceptive cutaneous afferents located in the epidermis. These methods include different types of laser and intraepidermal electrical stimulation techniques. The diffusion of the stimulation in the skin, inside or under the epidermis, depends on laser wavelength and electrode design, in particular. The aim of this study was to compare several of these techniques in their ability to selectively stimulate small nerve fibers.

METHODS

In 8 healthy subjects, laser stimulation (using a CO₂ or Nd:YAP laser) and intraepidermal electrical stimulation (using a micropatterned, concentric planar, or concentric needle electrode), were applied at increasing energy or intensity on the dorsal or volar aspect of the right hand or foot. The subjects were asked to define the perceived sensation (warm, pinprick, or electric shock sensation, corresponding to the activation of C fibers, Aδ fibers, or Aβ fibers, respectively) after each stimulation. Depending on the difference in the sensations perceived between dorsal (hairy skin with thin stratum corneum) and volar (glabrous skin with thick stratum corneum) stimulations, the diffusion of the stimulation inside or under the epidermis and the nature of the activated afferents were determined.

RESULTS

Regarding laser stimulation, the perceived sensations turned from warm to pinprick with increasing energies of stimulation, in particular with the Nd:YAP laser, of which pulse could penetrate deep in the skin according to its short wavelength. In contrast, CO₂ laser stimulation produced only warm sensations and no pricking sensation when applied to the glabrous skin, perhaps due to a thicker stratum corneum and the shallow penetration of the CO₂ laser pulse. Regarding intraepidermal electrical stimulation using concentric electrodes, the perceived sensations turned from pinprick to a combination of pinprick and electrical shocks with increasing intensities. Using the concentric planar electrode, the sensations perceived at high stimulation intensity even consisted of electric shocks without concomitant pinprick. In contrast, using the micropatterned electrode, only pinprick sensations were produced by the stimulation of the hairy skin, while the stimulation of the glabrous skin produced no sensation at all within the limits of stimulation intensities used in this study.

CONCLUSIONS

Using the CO₂ laser or the micropatterned electrode, pinprick sensations were selectively produced by the stimulation of hairy skin, while only warm sensation or no sensation at all were produced by the stimulation of glabrous skin. These two techniques appear to be more selective with a limited diffusion of the stimulation into the skin, restricting the activation of sensory afferents to the most superficial and smallest intraepidermal nerve fibers.

[The infrared laser in the diagnosis of normal and disturbed pain pathways.]

Cerebral potentials evoked by cutaneous heat stimuli from an infrared laser (LEP) enable overall controls of thin fibre function and anterolateral tract projection, which is of special meaning in the diagnosis of normal and disturbed pain pathways. Owing to the long-wave radiation, the laser energy is completely absorbed within the most superficial skin layers only a few 100 mum in depth and activates only the most superficial afferents, i.e. the thermo- and nociceptive A, delta and C fibres. According to the particular fibre spectrum activated, a single laser stimulus elicits a typical double pain sensation: the first pain appears with a mean reaction time of approximately 400 ms and is described as a sharp and stinging, well-localizable pinprick sensation; this pain is induced by A delta fibre activity with a mean conduction velocity of 14 m/s. It is followed by a second, more diffuse burning component, with a mean reaction time of 1300 ms, ascribed to selective C-fibre transmission (less than 1 m/s). These two pain sensations elicited by one laser-stimulus are accompanied by typical late and ultralate cerebral brain potentials. Both sensations and both evoked potentials can emerge to very different degrees in healthy subjects and in patients with neurological diseases. The diagnostic practicability of LEP is individuals illustrated with reference to patients with syringomyelia suffering from a dissociated sensory loss in cutaneous sensibility. In contrast to conventional electrical nerve stimuli, the laser stimuli allow monitoring of disturbances in the protopathic system projected to the anterolateral columns.

Pain processing is faster than tactile processing in the human brain

Pain signals threat and drives the individual into a behavioral response that significantly depends on a short stimulus-response latency. Paradoxically, the peripheral and spinal conduction velocities of pain are much slower than of tactile information. However, cerebral processing times and reaction times of touch and pain have not yet been fully assessed. Here we show that reaction times to selective nociceptive cutaneous laser stimuli are substantially faster than expected from the peripheral conduction velocities. Furthermore, by using magnetoencephalography, we found that latencies between earliest stimulus-evoked cortical responses and reaction times are approximately 60 ms shorter for nociceptive than for tactile stimuli. These findings reveal that cerebral processing of pain is substantially faster than processing of tactile information and relatively compensates for the slow peripheral and spinal conduction velocities of pain. Our observation shows how the cerebral organization of pain processing enhances motor responses to potentially harmful stimuli and thereby subserves the particular behavioral demands of pain.

Clinical application of laser evoked potentials using the Nd:YAG laser

The clinical interest of a new type of laser evoked potentials (LEPs) using Nd:YAG laser was assessed in the diagnosis of peripheral neuropathies affecting the small-diameter nerve fibres, and of spinal cord lesions, affecting the spinothalamic tract. Twelve patients aged from 26 to 79 years with sensory neuropathies (n = 6) or spinal cord lesions (n = 6) underwent neurophysiological examination of the lower limbs comprising quantitative sensory testing, i.e., the determination of vibratory and thermal thresholds (VT and TT), somatosensory evoked potentials (SEPs) to electrical stimulation and Nd:YAG LEPs. VT and SEPs were used to assess large-diameter afferent nerve fibres and the lemniscal pathways while TT and LEPs were used to assess small-diameter afferent nerve fibres and the spinothalamic tract. In addition, patients with peripheral neuropathy underwent also standard nerve conduction studies to explore large fibres and the recording of sympathetic skin responses (SSRs) to explore small fibres, whereas motor evoked potentials were performed in patients with spinal cord lesion. LEPs were absent bilaterally in all patients with polyneuropathy, even when TT remained within the normal limits and SSRs were present. LEPs were absent after stimulation of the affected limb in all patients with a spinal cord lesion, and allowed to detect subclinical contralateral lesion in two cases. LEPs following Nd:YAG laser stimulation are sensitive in the diagnosis of peripheral and/or central nervous system disorders and they give complementary information as compared to routine electrophysiological tests.

Scalp topography of ultralate (C-fibres) evoked potentials following thulium YAG laser stimuli to tiny skin surface areas in humans

AIM

To investigate (1) the scalp topography of ultralate laser evoked potentials (LEPs) related to C-fibre activation, which can directly be obtained by thulium YAG (Tm YAG) laser stimulation of tiny skin surface areas (about 0.23 mm(2)) and (2) the influence of the performance of a motor task on ultralate LEPs.

METHODS

Laser stimuli were applied to the dorsum of the left hand. LEPs were recorded with 58 scalp electrodes from 9 healthy subjects in two different conditions, with and without a reaction time (RT) task (press a button upon detection).

RESULTS

On high resolution electroenchephalogram recordings, ultralate LEPs were characterized by a broad positive component (peak latency: 1133+/-91 ms) with maximum amplitude about the vertex. Moreover, the performance of a RT task had no influence on latency, amplitude and topographical patterns of two maps chosen at the positive peak latency in ultralate LEPs. Nevertheless, a negative inflexion (latency 1300 ms) appeared after the positive component in the task condition possibly reflecting movement-related potentials.

CONCLUSION

Tm YAG laser stimulation of tiny skin surface areas allows recording the dynamic scalp topography of ultralate (C-fibres) LEPs, with or without the performance of a RT task.

CO2 laser activation of nociceptive and non-nociceptive thermal afferents from hairy and glabrous skin

In 8 healthy subjects we have recorded cerebral evoked potentials and reaction time (RT) to CO2 laser stimulation of the hairy and glabrous skin at low and high stimulus intensities, corresponding to subjective reporting of detection and pain, respectively. At each intensity we were able to identify an evoked potential; the latencies of the major vertex positive (VP) components fell into 2 distinct populations 320 +/- 30 (VP300) and 778 +/- 80 (VP800) which did not differ between stimulation sites. The frequency of the VP300 responses was greatest in the high stimulus conditions and lower in the low stimulus conditions whilst the opposite was true for the VP800 responses. BImodal distributions of RT were seen at both stimulus intensities. In a further group of of 10 subjects we recorded the latency shift of the vertex negativity following proximal and distal stimulation of hairy skin of the left upper limb and derived conduction velocities for the VP300 (13.21 +/- 2.8 m/sec) and VP800 (1.26 +/- 0.29 m/sec) responses. These results suggest that, following CO2 laser stimulation of both hairy and glabrous skin, two different fibre populations are activated. The VP300 responses appear to be related to A delta activation, while the characteristics of the VP800 responses are consistent with activation of thermoreceptors mediated by C fibres.

Influence of selective nerve fiber blocks on argon laser-induced thermal pain in the human skin

Influence of selective nerve fiber blocks on cutaneous pain induced by short argon laser pulses was studied in healthy humans. The minimal energy per surface area needed to produce a pain sensation was lower with a larger stimulus surface indicating spatial summation. Both compression of the peripheral nerve innervating the stimulus site (A-fiber block) and prolonged topical treatment of the stimulus site with 1% capsaicin (a predominant block of nociceptive C-fibers) produced an elevation of the pain threshold. Prolonged capsaicin treatment produced significantly higher pain thresholds than the compression block. Magnitude of the pain threshold elevation induced by nerve blocks varied depending on the area of the stimulus surface. Especially following prolonged capsaicin treatment, pain threshold elevations tended to be smaller with the larger area of the stimulus surface suggesting an enhanced spatial summation effect. Tactile thresholds were significantly elevated only by the compression block. The results indicate that nociceptive C-fibers have a significant contribution to the threshold sensation of pain induced by argon laser. Moreover, following impairment of function in nociceptive C-fibers, the spatial summation of argon laser-induced pain tends to be enhanced.

The effect of age on A delta- and C-fibre thermal pain perception

It has been suggested that ageing may have a differential effect on C fibre-mediated protopathic/tonic pain versus epicritic/phasic pain perception mediated by A delta fibres. The present study attempted to independently assess age-related changes in the function of A delta- and C-nociceptive fibres by examining CO2 laser-induced thermal pain thresholds before, during and after a compression block of the superficial radial nerve in 15 young and 15 healthy elderly adult subjects. Nerve block efficacy was monitored via measures of cold, warm and mechanical threshold, and simple reaction time. During nerve compression block, reaction time and mechanical threshold increased, cold sensation became impaired while warm sensation remained unaffected throughout the test in both groups. With respect to pain sensitivity, young adults exhibited significant increases in thermal pain threshold during A-fibre block while pain threshold remained relatively stable across the 3 test periods in the elderly group. It would appear that elderly adults rely predominantly on C-fibre input when reporting pain whereas younger adults utilise additional input from A delta fibres. Subsequent analysis revealed that during pre- and post-block periods, older adults exhibited a significant elevation in thermal pain threshold; however, when A delta-fibre function was impaired and only C-fibre information was available, both groups responded similarly. These findings support the notion of a differential age-related change in A-fibre-mediated epicritic pain perception versus C-fibre-mediated protopathic pain.

Altered heat pain thresholds and cerebral event-related potentials following painful CO2 laser stimulation in subjects with fibromyalgia syndrome

A decrease in mechanical pressure pain thresholds, particularly over pre-designated tender points, is one of the defining characteristics of fibromyalgia syndrome (FS); however, changes in thermal pain sensitivity have not been investigated. The present study examined heat pain thresholds and cerebral event-related potentials following CO2 laser stimulation in 10 subjects with FS and 10 age-matched control volunteers. The results indicate that patients with FS exhibit a significant reduction in heat pain threshold when tested on the dorsal surface of the hand. In accordance with previous research, we also found a decrease in mechanical pain threshold over pre-designated tender points and at control sites as well as a significantly larger mechanically induced neurogenic flare response. These measures were highly correlated with thermal pain threshold even though different anatomical sites were stimulated. Hence, it seems likely that FS patients display a multimodal change in pain sensitivity which is generalized rather than anatomically restricted. Patients with FS also displayed a significant increase in the peak-to-peak amplitude of the cerebral potential evoked by CO2 laser stimulation at pain threshold intensity and 1.5 times pain threshold intensity. These findings suggest a greater activation of central nervous system (CNS) pathways following noxious input. Putative explanations for the increased CNS response are discussed, including mechanisms of peripheral nociceptor sensitization, altered CNS function and the role of psychological factors.

Central delay of the laser-activated rat tail-flick reflex

The latency of the heat-activated rat tail-flick (TF) reflex is dependent upon 4 variables, none of which has previously been determined: activation of cutaneous nociceptors (TN); afferent conduction to the dorsal horn (TA); conduction within the central nervous system (CNS) (central delay); and conduction from the ventral horn (VH) to, and activation of, tail muscles (TE). Using a CO2 infrared laser (10 W, 45 msec) to produce synchronous activation of tail-skin nociceptors, TF latency (EMG response) was measured in 10 awake rats. Based on shifts in response latency from points of stimulation near the tip and base of the tail, conduction velocity in the afferent limb of the reflex was estimated to be 0.76 +/- 0.11 m/sec. This indicates that the response is mediated by C fibers. The rats were then anesthetized with pentobarbital and multiple-unit activity and evoked potentials (EPs) were recorded from the superficial dorsal horn at spinal segments S3-CO1 during laser or high-intensity electrical (10 mA, 1 msec) stimulation of the tail. Unit activity and EPs elicited by both stimuli consisted of two distinct components, corresponding to activation of A and C fibers. The difference in latency between laser and electrical evoked activity indicated that 60.00 +/- 7.33 msec was required for activation of nociceptors by the laser. Electrical stimulation of the VH at S3-CO1 in 3 rats produced a TF (EMG) response in 4 msec. Central delay, calculated as total TF time minus (TN+TA+TE), was 82.3 +/- 13.08 msec. This represents the time frame during which modulation of the reflex by an intrinsic, pain-activated, supraspinal system could occur.

Reaction times to painless and painful CO2 and argon laser stimulation

The introduction of lasers in pain research has made it possible to activate the nociceptive system without activating mechanosensitive afferents. In the present study the reaction times to painless and painful laser stimuli were studied to investigate if the reaction time to experimental pain is reproduceable. CO2 and argon lasers were used for stimulation, and the influence of stimulus (intensity and duration) and skin parameters (temperature, thickness, and reflectance) on reaction time were investigated. When these parameters were controlled the reaction times to painful CO2 and argon laser stimulation were within the same range (350-450 ms), and the intra-individual variability minimal (6.9%). The reaction time was used to estimate peripheral conduction velocity (10 m.s-1) for the activated fibre population when distinct pain was perceived. Determination of reaction times to non-painful and painful stimuli may be suitable ways to assess the functioning of thermal and nociceptive pathways.

Cutaneous pain and detection thresholds to short CO2 laser pulses in humans: evidence on afferent mechanisms and the influence of varying stimulus conditions

Pain and detection thresholds to short CO2 laser pulses were studied in healthy human subjects. Pain thresholds were significantly higher than detection thresholds in both hairy and glabrous skin; in the glabrous skin both thresholds were higher in the hairy skin. The range from detection threshold to pain threshold was larger in the glabrous skin. The minimal energy per surface area needed to produce any sensation (detection) or pain sensation decreased with increasing stimulus surface, and this spatial summation effect was to equal magnitude in the hairy and the glabrous skin. With decreasing stimulus pulse duration (from 45 to 15 msec) the detection and pain thresholds were elevated: this effect was stronger on pain thresholds. With increasing adapting skin temperature, less energy was needed to produce any sensation (detection) or pain sensation. The effect of adapting skin temperature was equal on pain and detection thresholds. The conduction velocity of fibers mediating laser evoked first sensations was in the thin fiber range (less than 10 msec), according to a reaction time study. The results suggest that short CO2 laser pulses produce both non-pain and pain sensations, but that both these sensations are based on the activation of the same primary afferent fiber population of slowly conducting nociceptive fibers. Central summation of primary afferent impulses is needed to elicit a liminal non-painful sensation, and an increased number of impulses in the same fibers produces pain.

Argon laser induced single cortical responses: a new method to quantify pre-pain and pain perceptions

The shape (amplitude and latency) of single cortical responses to argon laser stimulation was found to match six perceptual classes: three pre-pain and three pain. The amplitude of the pain related single cortical responses correlated with the perceived feeling of pain. Easy detectable responses were obtained because habituation to the stimuli was reduced and a high degree of attention was given to each stimulus. Single cortical responses to argon laser stimuli are suggested as a new quantitative technique with application in the assessment of function in the thermal and nociceptive pathways.

Effect of tourniquet-induced ischemia on cutaneous thermal thresholds

The effect of tourniquet-induced ischemia on human thermal thresholds was studied. After the development of the A-fibre block (= a sharp elevation of cool threshold) the heat-pain threshold was still uninfluenced. This result supports previous evidence indicating that C-fibres mediate the liminal heat pain sensation. Thus, the quantitative determination of cutaneous heat pain thresholds provides a rather selective method for testing C-fibre mediated pain sensitivity, at least when a contact thermostimulator with a slow or moderate rise of stimulus temperature is used. The second aim of this study was to examine whether ischemia or mechanical pressure is the cause of the tourniquet-induced block of A-fibres. This was studied by varying the mechanical pressure and the amount of ischemia. With increased ischemia (with muscle work) the A-fibre block (increased cool threshold) came earlier, but this finding was not significant.

Influence of the rate of temperature change on thermal thresholds in man

Thermal thresholds (cool, warm, heat, heat pain) were determined in four skin regions (cheek, glabrous skin of the hand, hairy forearm, leg) of eight healthy human subjects. The thermostimulator was composed of Peltier elements and three rates of continuous stimulation were used: 1.4, 2.4, and 3.9 degrees C/s. Warm, heat, and heat pain thresholds increased with increasing rate of temperature change, and the increase was of equal magnitude with these three thresholds. However, the effect of increasing stimulus rate on cool thresholds was nonsignificant. Similar results were obtained in all skin regions studied. It is suggested that liminal warm, heat, and heat pain sensations are mediated by afferent fibers with conduction velocities of the same range (C-fibers) whereas liminal cool sensations are signaled by faster conducting afferent fibers.